When Italy shut the Leaning Tower of Pisa to the public in 1990, the famous tilt had passed 5.5 degrees. The top of the 800-year-old marble campanile hung roughly 15 feet beyond its base, and the first storey’s masonry was nearing the point where towers can crack or burst under stress.
A commission of 13 experts, led by soil mechanics professor John Burland, got a clear assignment, save the tower without making it perfectly vertical. The lean is the monument’s identity, and the goal was stability, not a straightened rebuild.
The root problem sat under the masonry. Started in 1173 on a floodplain, the roughly 14,000-ton tower stands on a foundation about 3 meters deep set into soft silt and clay. Uneven soil conditions meant the tower began settling south before construction even reached the third storey. Later builders compensated by subtly adjusting the upper floors, creating the tower’s banana-like curve.
Past fixes showed what not to do. In 1934, grout was pumped into the foundation, and the tower leaned further south. Burland’s team spent years testing ideas, including counterweights and anchors. The solution that held was soil extraction, a controlled removal of ground from beneath the north edge of the foundation, the side opposite the lean. Beginning in 1999, engineers drilled 41 closely spaced holes and removed dozens of tons of soil in small increments, tracking movement day by day with instruments. Steel cables served as backup restraint, and drainage addressed another driver, rain pushed up the north side water table and aggravated seasonal movement.
By 2001, the tower had shifted back about 44 centimeters, or 17 inches, returning it to its 1830s inclination. Stress in the masonry dropped, the lean still read as “Pisa,” and engineers projected stability for the next 300 years. Monitoring later showed the tower continued a slow northward settle for more than a decade, adding about 4 centimeters before stabilizing.
For mason contractors and restoration teams, Pisa is a reminder that historic masonry problems are not always solved with more grout, more steel, or more intervention in the wall. Start with diagnosis, include geotechnical expertise early, treat drainage and water movement as structural issues, and use incremental work paired with monitoring when the building’s fabric has to stay untouched.
Read the full, original article from ScienceBlog.com here.